The production of an electronic component such as a multilayer capacitor or a ceramic multilayer substrate includes a step of forming an electrode. A method for forming this electrode is one based on screen printing or gravure printing, in which an electroconductive paste that contains a metal powder as an electroconductive component is applied to a ceramic or similar device section to form an electroconductive paste film, and this electroconductive paste film is heat-treated to form the electrode.
Such a screen printing or gravure printing method is suitable for mass production, but the use of a printing plate makes this method unsuitable for high-mix low-volume production and the formation of an electrode in a complicated pattern. Thus inkjet printing has been attracting attention as a new printing method.
Inkjet printing cannot be performed using an electroconductive paste or an electroconductive ink in the same way as in screen printing or gravure printing. Examples of proposed electroconductive inks that can be used in inkjet printing include the following.
Japanese Unexamined Patent Application Publication No. 2000-327964 (Patent Document 1) proposes an organic-solvent-based electroconductive ink containing a metal powder having a particle diameter of 10 μm or less and having a viscosity of 200 mPa·s or less with a sedimentation of 10 mm or less in 10 minutes or 20 mm or less in 100 minutes.
Japanese Unexamined Patent Application Publication No. 2002-121437 (Patent Document 2) proposes an organic-solvent-based electroconductive ink containing a metal particle having a particle diameter of 100 nm or less, preferably 10 nm or less, and having a viscosity of 1 to 100 mPa·s and a surface tension of 25 to 80 mN/m.
Japanese Patent No. 4303715 (Patent Document 3) proposes an organic-solvent-based electroconductive paste, although this paste is not intended for inkjet printing. Besides containing an electroconductive particle, this paste contains a particle that is based on the same main component as a dielectric ceramic material that forms the ceramic body of a ceramic electronic component to which this paste is applied, i.e., a material-matched (dielectric) particle. The electroconductive particle has a particle diameter of 10 to 100 nm, the ratio of the particle diameter of the electroconductive particle to that of the material-matched particle is in the range of 0.1 to 0.25, and the amount of addition of the matching material is in the range of 5% to 30% by mass per 100% by mass of the electroconductive particle. As presented in Table 4 of Patent Document 3, the smallest particle diameter of the material-matched particles used in Examples was 30 nm. The publication does not show any data obtained with a smaller diameter of the material-matched particle.
Japanese Unexamined Patent Application Publication No. 2006-28320 (Patent Document 4) proposes an aqueous metal ink for inkjet printing having a viscosity of 35 mPa·s or less. Unlike organic-solvent-based inks, aqueous metal inks contain ionic resins as a dispersant and a binder. The metal particle contained in the aqueous metal ink described in Patent Document 4 has a particle diameter of 50 to 400 nm. Although this aqueous metal ink also contains a matching material (a barium titanate powder) as a constituent, Patent Document 4 mentions nothing about the range of the particle diameter of the matching material. In Examples, a barium titanate powder having a particle diameter of 200 nm is the only disclosed matching material.
When producing a ceramic electronic component and trying to form an electrode using inkjet printing, however, one faces the following problems with the technologies described in Patent Documents 1 to 4.
The electroconductive ink described in Patent Document 1 has sedimentation characteristics that allow for a sedimentation of 10 mm or less in 10 minutes or 20 mm or less in 100 minutes. With such a level of sedimentability, it is difficult to form a printed film with uniform thickness in a stable manner because the applied ink often varies in thickness as a result of sedimentation.
An ink that contains a metal particle having a particle diameter that falls within a range of the particle diameter of a metal particle like that described in Patent Document 2, 100 nm or less, e.g., an ink containing a metal particle having a particle diameter of less than 50 nm, forms a colloid. As a result, the metal particle settles more slowly than one having a larger diameter. With such an ink it is relatively easy to form a printed film with uniform thickness in a stable manner. With an ink that contains a metal particle having a particle diameter in the range of 50 to 100 nm, however, it is difficult to form a printed film with uniform thickness in a stable manner because the applied ink often varies in thickness as a result of sedimentation. Making the particle diameter 50 nm or more often causes problems of seriously blurred printed dots and graphics as well. Furthermore, a printed film formed using an ink that contains a metal particle having a particle diameter of 100 nm or less has a very low heat shrinkage temperature compared with a ceramic layer. This means that differential shrinkage occurs during firing, often causing cracks and other structural defects in the ceramic electronic component.
The electroconductive paste described in Patent Document 3 does not have a rheology suitable for inkjet printing. As far as in the Examples described in Patent Document 3, what is disclosed is an electroconductive paste for screen printing. If this paste were given a viscosity that allows for inkjet printing, making the diameter of the electroconductive particle 50 nm or more would often cause problems of seriously blurred printed dots and graphics. Furthermore, the electroconductive paste described in Patent Document 3, in which the ratio of the diameter of the electroconductive particle to that of the material-matched particle is too high, ranging from 0.1 to 0.25, experiences a significant sedimentation of the electroconductive particle where the diameter of the electroconductive particle is 150 nm or more. With this paste, therefore, it is difficult to form a printed film with uniform thickness in a stable manner.
The metal ink described in Patent Document 4, which is an aqueous ink, is extremely slow to dry. In such a case, significant printing bleeds and a long time needed for the ink to dry cause the problem of limited printing speed. The use of a ceramic sheet that has an increased number of pores for water absorbency like ordinary inkjet paper solves the bleeding issue, but at the same time seriously shrinks when fired, often causing structural defects in the ceramic electronic component. Furthermore, an aqueous ink is unsuitable for printing on a ceramic electronic component because of limited options of usable binders.
In producing a multilayer ceramic electronic component, the following problem can be encountered. That is, when an electroconductive film to serve as an inner electrode is formed on a ceramic green sheet, a height gap resulting from the thickness of this film accumulates as multiple ceramic green sheets are stacked. The accumulated height gap can cause delamination of the multilayer body or any similar situation, and can also cause cracks and other structural defects during a firing step. To solve this problem, Japanese Unexamined Patent Application Publication No. 9-219339 (Patent Document 5) proposes forming a leveling layer for compensating for a height gap resulting from the thickness of an inner electrode by applying a ceramic ink through inkjet printing.
Japanese Unexamined Patent Application Publication No. 2005-223323 (Patent Document 6) describes an ink composition that can be applied to a substrate through inkjet printing. This ink composition contains (a) a functional material, (b) an organic polymer containing polyvinyl pyrrolidone, and (c) a dispersion vehicle selected from an organic solvent, water, or their mixture, with the functional material and the organic polymer dispersed in the dispersing vehicle. The viscosity of this ink composition ranges from 5 mPa·s to 50 mPa·s at temperatures of 25° C. to 35° C. The functional material is a material having a function such as electroconductivity, resistance, or dielectricity. The publication lists materials such as a variety of metals and barium titanate as specific examples. Paragraph [0026] of Patent Document 6 states that it is preferred that the D50 particle diameter of the functional material be in the range of 0.005 μm to 2 μm.
When containing a metal as the functional material, the ink composition described in Patent Document 6 can be used to form an electrode in a ceramic electronic component through inkjet printing. When containing a ceramic material (a resistor or a dielectric) as the functional material, this ink composition can be used to form, through inkjet printing, a leveling layer for compensating for a height gap that occurs in a multilayer ceramic electronic component as a result of the thickness of an inner electrode.
As detailed below, however, the ink composition described in Patent Document 6 also has problems that need to be solved.
When the functional material is a metal, problems similar to the aforementioned ones with the technologies described in Patent Documents 1 to 3 can be encountered.
Likewise, when the functional material is a ceramic material and the ink composition is used to form a printed film for a leveling layer in a multilayer ceramic electronic component, problems nearly similar to the aforementioned ones with the technologies described in Patent Documents 1 to 3 can be encountered. This means that when the particle diameter of the functional material is 50 nm or more, significant sedimentation makes it impossible to form a coating with uniform thickness in a stable manner through printing, and problems of seriously blurred printed dots and graphics occur. When the particle diameter of the functional material is less than 50 nm, the ink composition forms a colloid, making the particle less likely to settle. This particle diameter is, however, much smaller than the pre-firing BET-equivalent particle diameter (100 nm or more) of a particle that is the main component of the ceramic section of an ordinary ceramic electronic component. The subsequent relatively large differential shrinkage that occurs during firing causes cracks and other structural defects in the ceramic electronic component.
An inkjet ink must therefore have a low viscosity than that of a known ink or paste for screen printing or gravure printing. The use of an ink that falls within such a low viscosity range and contains a functional particle (an electroconductive particle or a ceramic particle) having a particle diameter of 50 nm or less, however, suffers from problems of the sedimentation of the metal particle and seriously blurred printed dots or graphics. Making the particle diameter of the functional particle less than 50 nm often causes structural defects to occur while the ceramic electronic component is fired.
An inkjet ink containing a particle having a particle diameter of 50 nm or more therefore need to have all of three characteristics consisting of viscosity characteristics that allow for inkjet printing, sedimentation characteristics that ensure stable printing, and the nature of being unlikely to bleed after printing. When applied to a ceramic electronic component, this inkjet ink must have heat shrinkage characteristics that allow the ink to shrink without causing structural defects, besides the above three characteristics.
In addition, the sedimentation issue is relatively minor for an aqueous ink because the electrostatic and steric repulsions exercised by ionic resins stabilize the particle in a dispersed state. As for an organic ink, which is suitable for printing on a ceramic electronic component, the sedimentation issue is very significant because no electrostatic repulsion occurs.
Stokes' law below is known as a classic equation for the rate of sedimentation of a particle:νs={Dp2(ρp−ρf)g}/18η
(where νs is the rate of sedimentation of the particle, Dp is the diameter of the particle, ρp is the specific gravity of the particle, ρf is the specific gravity of the dispersion medium, g is the gravitational acceleration, and η is the viscosity of the dispersion medium).
Stokes' law indicates that in an organic ink in which a particle having a particle diameter of 50 nm or more is dispersed, increasing the viscosity η of the dispersion medium effectively reduces troublesome sedimentation of the particle.
In general, η in Stokes' equation is construed as a spatial viscosity (the viscosity of the vehicle system). This leads to the conclusion that η can be increased only through an approach that makes viscosity develop only in a space, such as increasing the viscosity of the solvent, adding a thickening resin, or adding a thixotropic agent. However, increasing the spatial viscosity to a level high enough to reduce the sedimentation of the particle causes the user to encounter a problem of the inability to apply the ink through inkjet printing because the ink remains highly viscous (has not sufficiently lost its viscosity) even at a moment of inkjet printing, at which a high shear force is applied.
It is therefore very difficult to solve the sedimentation issue with an organic ink containing a particle having a particle diameter of 50 nm or more while ensuring viscosity characteristics that allow for inkjet printing.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-327964
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-121437
Patent Document 3: Japanese Patent No. 4303715
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-28320
Patent Document 5: Japanese Unexamined Patent Application Publication No. 9-219339
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2005-223323